A conventional voltage conversion device includes an up-converter and a control device. The control device receives an output voltage of the up-converter, a torque control value to be output by an alternating-current (AC) motor as a load, and a rotation number of the AC motor.
The control device calculates a target voltage of the up-converter in accordance with the torque control value and the motor rotation number, and further calculates an error between the calculated target voltage and the output voltage of the up-converter. The control device then adjusts PI control gain (proportional gain and integral gain) according to the calculated error, and uses the adjusted PI control gain to provide feedback control to the up-converter.
Under the feedback control by the control device, the up-converter converts a direct-current (DC) voltage from a DC power supply into the output voltage such that the output voltage attains the target voltage (Japanese Patent Laying-Open No. 2003-309997).
In this manner, the conventional voltage conversion device adjusts the PI control gain in accordance with the error between the target voltage and the output voltage converted from the DC voltage, and uses the adjusted PI control gain to convert the DC voltage from the DC power supply into the output voltage such that the output voltage attains the target voltage.
In the conventional voltage conversion device, however, when the output voltage deviates from the target voltage and a duty ratio is considerably changed to allow the deviated output voltage to reach the target voltage, electric power returning back to the power supply side is increased and an overvoltage is applied to the power supply.
An object of the present invention is to provide a voltage conversion device converting a voltage without applying an overvoltage to a power supply.